In the ongoing evolution of advanced radio systems, carrier aggregation has been considered as one possibility to meet the backwards compatibility requirements for example in one realization of a Long Term Evolution Advanced (LTE-A). LTE-A is the next step from LTE, fulfilling the requirements of the fourth generation (4G) communication network as specified by the International Telecommunications Union (ITU). LTE is also the next step from a universal mobile telecommunications system (UMTS).
Some of the main requirements related to the backwards compatibility are for example: a Release 8 E-UTRA (enhanced UMTS terrestrial radio access) terminal must be able to work in an Advanced E-UTRAN (enhanced UMTS terrestrial radio access network), and an advanced E-UTRA terminal must be able to work in a Release 8 E-UTRAN.
LTE-A applies a physical uplink control channel (PUCCH) to transmit control signals, such as an acknowledgement (ACK)/negative-ACK (NAK), a channel quality indicator (CQI) and a scheduling request (SR) indicator, from user equipment (UE) to an evolved node B (eNB). There are two alternative ways to transmit uplink control signals in LTE-A: (1) PUCCH and (2) PUSCH (physical uplink shared channel) time-multiplexed with uplink data. This application deals mainly with uplink control signals on the PUCCH. From the point of view of uplink/downlink control signalling, one solution is to copy the existing Release 8 control plane (PDCCH, PUCCH etc.) to each component carrier (CC). From now on, this concept is denoted as a NxPDCCH structure in LTE-Advanced. Due to the backwards compatibility requirement, it is also assumed that Release 8 type PUCCH resources are reserved for each downlink component carrier transmitting PDCCH. Those resources are located on the corresponding uplink component carriers.
One baseline assumption for LTE-Advanced has been to support one transport block and HARQ (hybrid automatic repeat request) entity per component carrier. It is generally understood that having one separate PDCCH per component carrier (NxPDCCH) seems to be a suitable downlink control signalling approach for such system operation. From the point of view of the uplink control signalling, there are certain aspects that require special attention when using the NxPDCCH approach. One aspect is cubic metric (CM) properties. Multi-carrier transmission is always realized in uplink when uplink/downlink resources are allocated into different component carriers. From uplink point of view, single carrier transmission should be the target whenever possible to minimize the CM, i.e. the simultaneous transmission of parallel PUCCHs (NxPUCCH) should be avoided. Another aspect is control channel coverage in uplink. Multi-ACK/NAK transmission (ACK/NAK multiplexing) is always realized when more than one downlink component carrier is allocated. Uplink coverage is an issue with multi-bits ACK/NAK. Therefore, ACK/NAK bundling, i.e. one common ACK/NAK for all downlink transport blocks and allocated component carriers, should always be an option to ensure optimized uplink coverage. Thus, more advanced control channel design solutions are needed to support ACK/NAK bundling on the PUCCH.